This invention generally relates to an electro-optical scanning system for reading symbols, especially bar code symbols and, more particularly, to non-laser-based scanners operative for focusing a light beam and the view of a light sensor in different planes exteriorly of a scanner housing.
It has heretofore been proposed to read bar code symbols, particularly of the Universal Product Code (UPC) type, by using laser and non-laser scanners of the type disclosed in, for example, U.S. Pat. Nos., 4,251,798; 4,387,297; 4,409,470; 4,806,742 and 4,825,057, all of which have been assigned to Symbol Technologies, Inc., the assignee of this invention, and are hereby incorporated by reference herein.
Typically, a laser beam generated by a laser source, for example, a gas laser tube or a semiconductor laser diode, is optically focused by an optical train into a generally circular laser beam spot on a symbol. The beam spot is swept by a scanning component over the symbol and forms a scan pattern thereon. Laser light reflected off the symbol is detected by a light sensor, e.g. a photodiode, mounted together with the laser source, the optical train, the scanning component, and the photodiode in a housing, preferably one having a handle to enable hand-held, portable operation.
The symbol itself is a coded pattern comprised of a series of bars of various widths, the bars being spaced apart from one another to bound spaces of various widths, the bars and spaces having different light-reflective properties. Although dimensions may vary, depending on the particular application and the density of the symbol, each bar and space of a UPC symbol typically used in the retail industry to identify retail products measures on the order of thousandths of an inch (mils). In practice, the generally circular laser beam spot has a cross-sectional diameter on the order of 6 to 10 mils.
Although the known laser scanners have enjoyed considerable commercial success, there is nevertheless incentive to reduce the cost of the scanner unit. The laser devices produce a very intense light spot of small size, and thus have inherent advantages. However, the laser light sources are of relatively high cost compared, for example, to non-laser sources such as light emitting diodes (LEDs). The use of non-laser sources presents problems, since it is difficult to focus a non-collimated LED source to beam spot sizes measuring on the order of mils, at least not without resorting to expensive, heavy, multiple-element optical trains or loss of power. LEDs can typically be focused to spot sizes on the order of millimeters. However, using such a large-sized beam spot to read bars and spaces which measure on the order of mils imposes a significant burden on the signal processing and decode circuitry for the scanner. Non-reads and reading errors are likely.
By contrast, in laser-based systems, where the laser beam spot dimensions are on the same order of magnitude as those of the bars and spaces to be decoded, the signal processing and decoding circuitry has no such burden. The photodiode in such laser-based systems typically "looks" at a large volume of space surrounding the beam spot and in a common plane therewith.